Both fiber amplifiers and Raman amplifiers, in which the optical transmission fiber is pumped with a high optical power in the opposite direction to the transmission signal, are used in modern optical networks. The resultant quantum-mechanical effect amplifies the transmitted signal whose wavelength is above the pump wavelength. This concept allows the signal-to-noise ratio to be considerably improved and allows the amplifier-free/regenerator-free transmission length to be increased.
As a result of the use of Raman amplifiers, the Raman pump laser must be switched off in an even more reliable manner than the transmission lasers in the event of the fiber being interrupted in order to avoid endangering people by the pump light emerging from the interrupted fiber.
For signaling purposes, a service signal is transmitted in addition to a wavelength-division multiplex signal. Said service signal may be modulated onto the complete WDM signal, for example. Nowadays, the service signal is usually transmitted in a separate service channel whose wavelength usually has a relatively large wavelength separation from the data channels. In this case, the service signal is fed in with a relatively low level so that it can always be transmitted, that is to say even in the case of an interrupted line, without endangering people. Reception of the service signal is detected, is rated as proof of an intact connection and is used to switch on the Raman pump laser and the transmission lasers, whereupon data transmission is then started or continued.
In the case of transmission paths of the maximum length and when the Raman pump laser is switched off, in particular, detection of the service signal is problematic if its level is below the noise level and it is impossible to regenerate the data in the service channel.
The patent application WO 03/088528 A1 describes, as the closest prior art, a method for detecting a control signal. A small proportion of power is branched off from the received signal using a coupler, is optoelectrically converted and amplified, and a spectral line of the control signal is then selected. The power of the isolated spectral line is assessed and is used to switch the Raman pump laser on and/or off. In this case, the service channel is coded in such a manner that a relatively large proportion of the transmission signal power is concentrated on a spectral line which corresponds to the clock frequency. CMI coding or transmission of RZ pulses is used, for example, for this purpose.
However, this solution becomes more inefficient as the data rate of the service channel increases since a considerable proportion of the power of the service signal is used for the signal power at the “clock line” rather than for transmitting information. A “penalty” of approximately 2.5 dB can be expected with this type of coding, which penalty can be compensated for by a correspondingly higher power of the laser at the transmitting end with a correspondingly higher price, but the maximum permissible power of the service signal must not be exceeded.
The work entitled “Low-Jitter Symbol Timing Recovery for M-ary QAM and PAM signals” by Afshin Haghighat in order to attain the “Degree of Master of Applied Science” from Concordia University Montreal, Quebec, Canada, August 1998, 0-612-39476-X, pages 17, 18 and 45, describes the recovery of the clock signal by means of high-pass prefiltering, squaring and narrowband bandpass filtering. The aim is to recover the clock signal, for which the higher frequency components of the spectrum are required, but, owing to the high-pass filter, the noise above the Nyquist frequency is not limited.